How Do Lie Detectors Work

How polygraph lie detectors work

After seeing lie detectors on television, many people wonder how a device can really root out deception. The truth is that lie detectors, or polygraphs, don’t determine truth or lies at all, they merely provide data to a polygrapher about what’s going on with a person’s body. Lying inevitably leads to certain changes in body language and physiological responses, and a polygraph helps to find those changes and signal that a lie is taking place.

Polygraphs have sensors that measure three basic physiological responses: heart rate, blood pressure, and perspiration. When someone is lying, their anxiety rises causing disruptions to the normal levels for these three areas.

A Polygraph session is built around the polygrapher establishing the normal levels of activity for the person being examined and then comparing those to how their levels rise when asked questions about whatever has brought to the polygraph.

The sessions begin by interviewing the examinee to learn about them generally, asking questions about their life and gathering basic facts. They will address whatever topic has brought them to the polygraph, whether it is a crime, a job application, or a potential infidelity.

The polygrapher then constructs a series of about ten questions. Some of these are “control” questions that are about the examinee’s life in general and others are “relevant” questions about the topic at hand. All the questions are can be answered with yes or no, and the polygrapher then looks at the data coming in from the sensors, comparing their stress level in answering control questions versus relevant questions.

This comparison is what leads the polygrapher to make an educated guess about whether the person was being honest, or telling a lie. A lot of the accuracy of a polygraph is about the skills of the polygrapher and their ability to construct the bet questions and make proper readings of the person being examined. That’s why it’s important to always choose a polygrapher who has been certified and professionally trained.

Are polygraph results accurate?

The accuracy of the polygraph test is often disputed, but many professional associations have found that polygraphs are accurate 90 percent of the time. For law enforcement, pre-employment screening, and anyone who wants to put somebody else’s story to the test, a polygraph remains the best and the only device available that can help you determine whether or not someone is lying.